This invention relates to mechanically steerable antennas, suitable for use on board a communications satellite encircling the earth and, more particularly, to an antenna having an array of feed elements illuminating a reflector wherein the reflector is a shaped-beam reflector and wherein, upon a repositioning of the antenna relative to the satellite, there is an adjustment of the beam configuration by use of a feed network comprising variable power dividers and variable phase shifters.
In the use of communication satellites, one or more antennas on board the satellite direct beams of radiation to selected locations on the earth's surface as the satellite progresses in a stationary orbit about the earth. Based on considerations of communication traffic, it may be desirable to reorient an antenna relative to the body of the satellite so as to illuminate another portion of the earth's surface. Each portion of the earth's surface to be illuminated is characterized by a desired beam footprint designating the specific area of the earth's surface wherein are located receiving and/or transmitting stations which are to communicate with the satellite via the antenna. The antenna includes both a reflector and an array of feeds which illuminate the reflector to produce a beam configuration and corresponding footprint. However, with a repositioning of the antenna to illuminate different regions of the earth, it is desirable frequently to adjust the configuration of the footprint to meet local traffic conditions.
By way of example, two antennas on board a single satellite may be directed to illuminate various areas of the earth's surface in footprints configured to overlap edge regions of neighboring footprints. Or, antennas carried by different satellites may cooperate by illuminating various areas of the earth's surface wherein footprints from the beams of the various antennas are to overlap slightly at the peripheral regions of the respective beam footprints. The configurations of the various footprints may vary, depending on the viewing angles by which the satellite directs the beam to the earth. The footprints may vary such that a relatively small populated region of the earth may be assigned to one beam. A much larger region of the earth of relatively sparse population may employ a single antenna covering the larger region. Therefore, upon a repositioning of a beam, it may be necessary to provide for adjustment of the beam configuration.
It is desirable to be able to adjust the footprint in a generally continuous fashion so as to allow for various configurations of footprints which may become desirable as a result of shifting patterns of population on the earth and shifting demands for the satellite service. Prior communication antennas have been inadequate in this respect in that their capacity for alteration of footprint configuration was limited to a relatively small number of footprints obtainable by switching various feeds of a feed network.
A problem exists in that presently available communication antennas while being mechanically steerable do not provide a desired capacity for precise adjustment of footprint configuration.